Head suspensions are well known and commonly used within dynamic magnetic or optical information storage devices or drives with rigid disks. The head suspension is a component within the disk drive that positions a magnetic or optical read/write head over a desired position on the storage media where information is to be retrieved (read) or transferred (written). Head suspensions for use in rigid disk drives typically include a load beam that generates a spring force and that supports a flexure to which a head slider having the read/write head is to be mounted. The load beam includes a mounting region at a proximal end, a rigid region at a distal end, and a spring region between the rigid region and the mounting region for providing the spring force. Head suspensions are normally combined with an actuator arm or E-block to which the mounting region of the load beam is mounted with a base plate so as to position (by linear or rotary movement) the head suspension, and thus the head slider and read/write head, with respect to data tracks of the rigid disk.
The rigid disk within a disk drive rapidly spins about an axis, and the head slider is aerodynamically designed to “fly” on an air bearing generated by the spinning disk. The spring force (often referred to as the “gram load”) generated by the load beam (when the load beam is at its “fly height”) urges the head slider in a direction toward the disk opposing the force generated by the air bearing. The point at which these two forces are balanced during operation is the “fly height” of the head slider.
Another important attribute of head suspensions is referred to as radius geometry height or “RG height.” This attribute or characteristic of the head suspension is the offset or distance that specific points or area of the rigid portion of the load beam of the suspension are displaced from a mounting plate in the mounting region of the suspension. RG height is an important characteristic to control, because it affects the torsional resonance characteristics of the load beam and thus affects overall performance of the head suspension assembly.
In addition to “RG height,” the load beam may be characterized by a parameter referred to as Delta Radius Geometry or ARG or “Delta Height.” This parameter measures the twist of the load beam. It is measured by finding the difference between RG height measured on each of two radius legs or arms. A twisted load beam can result in a suspension with poor resonance characteristics, in addition to possible distortion in flying attitude of the head slider. The twist affects windage, bending modes and torsional modes of the suspension. The “Delta Height” is thus seen to be the incremental height between the pair of legs in the spring region of the suspension.
The flexure typically includes a slider bond pad to which the head slider is attached. The flexure provides a resilient connection between the head slider and the load beam, and permits pitch and roll motion of the head slider and read/write head during movement over the data tracks of the disk in response to fluctuations in the air bearing caused by fluctuations in the surface of the rigid disk. The roll axis about which the head slider gimbals is a central longitudinal axis of the head suspension. The pitch axis about which the head slider gimbals is perpendicular to the roll axis. That is, the pitch axis is transverse to the longitudinal axis of the load beam, and crosses the roll axis at or around the head slider.
In order to store and retrieve data from magnetic or optical disks on which data is densely packed, it is necessary for the head slider to fly closely above the surface of the spinning data disk (on the order of 0.1 μm) without colliding with the disk (“crashing”). Further, because of the dense packing of data on magnetic or optical disks, it is important for the read/write head attached to the head slider to be able to read from or write to a relatively small area or spot on the disk.
The gram load provided by the spring region of the load beam is transferred to the flexure via a dimple or other pivoting structure that extends between the rigid region of the load beam and the flexure. The structure that extends between the rigid region of the load beam and the flexure is referred to herein as the “gimbal region” to distinguish from the “spring region” which, as used herein, refers only to the region adjacent a rigid mounting region, usually between the mounting region and the rigid region of the load beam. In the prior art, the spring region (which in some instances may be part of the load beam) was mechanically deformed, using a process of roll bending to impart a curvature to at least a portion of the spring region, typically moving the load beam out of plane from the mounting region (where, typically, a base plate was attached).
The prior art also included adjusting the spring region using a diode laser to thermally bulk heat the load beam in the spring region while the load beam was mechanically moved in an attempt to “trim” or correct radius geometry height. Also, reverse bending or “back bending” typically occurred in the process of manufacture of the head suspension, where the part was intentionally bent or plastically deformed one time, in a direction opposite to and after the rolling process is applied to the part to form the spring region. When the part was subsequently integrated into a head stack assembly, the suspension is “bent back” again to fit the heads between the discs. Lack of care in this subsequent bending process has been known to plastically deform the part, resulting in an error in gram load known as “load loss” or more particularly, “suspension gram load loss.” The phenomenon of suspension gram load loss is to be distinguished from “gram creep” or more properly “elastic recovery instability.” The term “recovery instability” refers to the portion of the strain that remains immediately after the stress is removed and disappears after a period of time. The phrase “elastic recovery instability” refers to instability in the “recovery” after adjustment of the head suspension. The present invention overcomes a substantial amount of the elastic recovery.
Moreover, the prior adjustment method described above was typically performed using three stations, with the individual head suspension entering a first station dedicated to measuring the head suspension radius geometry height and gram load, then passing the individual head suspension to a second station in which adjustments were made, after which that particular head suspension was transferred to a third station which measured the head suspension again. If the radius geometry and gram load were not both within specifications as measured by the third station, the part was rejected, because cycle times would be unacceptably long if the part were to be recycled through the series of stations. Furthermore, such parts are typically carried in a strip of, e.g., 12 for ease of processing. If one or some, but not all, of the parts in a strip were out of spec, the in-spec parts would be subject to additional handling if the strip were to be “recycled” through the stations, thus exposing good parts to potential degradation because of the additional handling.
From the above, it can be seen that there has been a continuing need to develop more efficient methods for correcting the RG height (more particularly the radius geometry height) and for adjusting the gram load. A method that provides precise error corrections for both height and gram load in a timely fashion, and that can be achieved without significant impact on other performance criteria of the head suspension is highly desirable. It is further desirable to be able to adjust delta radius geometry, if necessary, to eliminate or at least reduce twist in the load beam.
Techniques for adjusting only the gram load of the head suspension assembly after it has been rolled are generally known and disclosed, for example, in the Girard U.S. Pat. No. 5,832,764 and the Schones et al. U.S. Pat. No. 5,297,413. Briefly, one such method is a laser adjust technique. A known property of stainless steel members such as load beams is that the force they exert in response to attempts to bend them can be reduced through exposure to thermal energy. The functional relationship between the amount of force reduction and the amount of heat to which a member is exposed can be empirically determined. The light adjust method makes use of this empirically determined relationship to “downgram” or lower the gram load of load beams that have been purposely manufactured (e.g., through rolling operations of the type described above) to have an initial gram load greater than the desired gram load value.
Equipment for performing the light adjust method includes a clamp for clamping the mounting region of the suspension to a fixed base or datum, and a load cell for measuring the gram load of the suspension. A computer controlled actuator moves the load cell into engagement with the flexure and elevates the flexure to a z-height or offset with respect to the datum which corresponds to the specified fly height for the suspension (i.e., the gram load is measured at fly height). In practice, the measured gram load quickly rises toward its then-current value as the flexure is elevated. When the measured gram load reaches an upper range specification, the computer actuates or turns on a high intensity light to apply heat to the load beam. Since the applied heat reduces the actual gram load of the suspension, the measured gram load quickly peaks. Continued application of laser energy causes the measured gram load to decrease with time. The computer deactuates or turns off the light when the measured gram load has decreased to a predetermined set point, typically a load between the nominal or desired gram load and the lower range specification. Once the light has been turned off, the decrease in gram load quickly slows and reaches its minimum value (often at a gram load below the lower range specification) as the heat in the suspension dissipates. However, as the load beam continues to cool, the measured gram load increases and stabilizes at an equilibrium or final load value that is preferably within the specification range, and ideally close to the nominal specification. The final gram load is also measured following the light adjust procedure. This measurement is used by a computer to update a stored model (e.g., the setpoint) of the functional relationship between the amount of heat applied (e.g., light “on” time) and the gram load reduction, to optimize the accuracy of the results obtained by the light adjust procedure. It is to be understood that the above described light adjust procedure does not use any scanning movement to accomplish its goal. The light source in the above procedure is thus a general energy source, applying heat generally to the spring region.
Computer controlled mechanical bending procedures have also been used to adjust the gram load on load beams. The mechanical bending method makes use of an empirically determined relationship between the amount that the load beam is mechanically bent and the associated change in gram load. For a range of gram load adjustments that are typically performed by this technique, a simple linear regression line has been found to accurately describe this relationship. In practice, this technique is implemented by a computer coupled to a stepper motor-driven bending mechanism and a load cell. A model of the relationship between changes in gram load and the number of motor steps (i.e., the associated amount or extent of bending required) is stored in the computer. After the then-current gram load of the suspension is measured by the load cell, the computer calculates the required load correction (i.e., the difference between the measured and desired loads). The computer then accesses the model as a function of the required correction to determine the number of motor steps required to achieve the required load correction, and actuates the stepper motor accordingly. Once the load beam has been bent, the then-current gram load is again measured and used to update the model. Measured data from a given number of the most recently executed mechanical bends is used to recompute the regression line data prior to the execution of the next mechanical bend.
The air bearing head slider assembly is mounted to the flexure and the lead wires clamped to the load beam after the gram load of the suspension has been initially set using methods such as those described above. Unfortunately, the mechanical handling and assembly procedures involved in this manufacturing operation sometimes forces the gram load of the assembled head suspension assembly beyond the specification range. Since the gram load specification is so critical to proper disk drive operation, these out-of-specification head suspension assemblies cannot be used unless the gram load is readjusted to the specification range. A machine which uses both light-adjust and mechanical bending procedures to “regram” suspensions is shown in the Schones et al. U.S. Pat. No. 5,297,413.
RG height and ΔRG are important because of its relationship to head suspension resonance characteristics. In order for the head slider assembly to be accurately positioned with respect to a desired track on the magnetic disk, the suspension must be capable of precisely translating or transferring the motion of the positioning arm to the slider assembly. An inherent property of moving mechanical systems, however, is their tendency to bend and twist in a number of different modes when driven back and forth at certain rates known as resonant frequencies. Any such bending or twisting of a suspension causes the position of the head slider assembly to deviate from its intended position with respect to the desired track. Since head suspension assemblies must be driven at high rates of speed in high performance disk drives, the resonant frequencies of a suspension should be as high as possible.
As discussed in the Hatch et al. U.S. Pat. No. 5,471,734, the position, shape and size of the roll or bend in the spring region of a suspension, sometimes generally referred to as the radius geometry or profile of the suspension, can greatly affect its resonance characteristics. The radius geometry of a suspension must therefore be accurately controlled during manufacture to optimize the resonance characteristics of the part.